1. Field of the Invention
The present invention relates to a vane type rotary pump for supplying hydraulic fluid under pressure to a hydraulically operated apparatus such as a power-assisted steering apparatus in an automotive vehicle.
2. Description of the Prior Art
A conventional vane type rotary pump of this kind is composed of a stator housing, a cam ring formed at its inner periphery with a cam surface radially offset from its central axis and mounted within the stator housing, a pair of end wall structures fitted to the opposite ends of the cam ring to form a pump cavity in the cam ring, a drive shaft rotatably mounted within the stator housing and extending into the interior of the pump cavity through one of the end wall structures, a rotor contained within the cam ring and mounted on the drive shaft for rotation therewith, and a plurality of circumferentially equally spaced vanes slidably fitted into the body of the rotor to move radially outward from the rotor and cooperating with the cam surface of the cam ring to form a plurality of expandable pump cambers. In the vane type rotary pump, one of the end wall structures is formed with a suction port at a portion where the pump chambers expand as the vanes move radially outward and is formed with a discharge port at a portion where the pump chambers contract as the vanes move radially inward.
In operation, the fluid compressed at the compression stroke suddenly changes in pressure when discharged into the discharge port. This causes pulsation of the fluid under pressure, resulting in the occurrence of vibration and unpleasant noises in the pump assembly. To solve such problems, there has been proposed a vane type rotary pump in Japanese Utility Model Laid-open Publication 57(1982)-30396, wherein one of the end wall structures 60 is formed at its inside face with a bearded groove 62 which is tapered from the discharge port 61 in a direction opposite to a rotational direction of the rotor as shown in FIG. 8 to gradually increase the pressure of fluid discharged into the discharge port. However, the bearded groove 62 is communicated with the discharge port 61 at a shoulder 63 of the end wall structure 60. With such a configuration of the bearded groove, the pressure in a pump chamber formed by adjacent vanes passing the suction port rapidly increases under a loaded condition of the rotary pump at a point of time shown by a character .theta.1 in FIG. 9(b), and the fluid under pressure in the discharge port 61 is introduced into the pump chamber through the bearded groove 62 immediately before the pump chamber is fully communicated with the discharge port 61. This causes an overshoot S' in pressure of the fluid shown in FIG. 9(b), resulting in a decrease S" of the pressure of fluid in a moment .theta.2 shown in FIG. 9(a) when the pump chamber was fully communicated with the discharge port 61. For this reason, the provision of the bearded groove does not effect to avoid pulsation of the hydraulic fluid pressure and to eliminate the occurrence of vibration and unpleasant noises in the pump assembly.